USB: usblp: don't call usb_set_interface if there's a single alt

Some devices, such as the Winbond Electronics Corp. Virtual Com Port
(Vendor=0416, ProdId=5011), lockup when usb_set_interface() or
usb_clear_halt() are called. This device has only a single
altsetting, so it should not be necessary to call usb_set_interface().

Acked-by: Pete Zaitcev <zaitcev@redhat.com>
Signed-off-by: Jeremy Figgins <kernel@jeremyfiggins.com>
Link: https://lore.kernel.org/r/YAy9kJhM/rG8EQXC@watson
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/class/usblp.c

@@ -1329,14 +1329,17 @@ static int usblp_set_protocol(struct usblp *usblp, int protocol)
 	if (protocol < USBLP_FIRST_PROTOCOL || protocol > USBLP_LAST_PROTOCOL)
 		return -EINVAL;
 
-	alts = usblp->protocol[protocol].alt_setting;
-	if (alts < 0)
-		return -EINVAL;
-	r = usb_set_interface(usblp->dev, usblp->ifnum, alts);
-	if (r < 0) {
-		printk(KERN_ERR "usblp: can't set desired altsetting %d on interface %d\n",
-			alts, usblp->ifnum);
-		return r;
+	/* Don't unnecessarily set the interface if there's a single alt. */
+	if (usblp->intf->num_altsetting > 1) {
+		alts = usblp->protocol[protocol].alt_setting;
+		if (alts < 0)
+			return -EINVAL;
+		r = usb_set_interface(usblp->dev, usblp->ifnum, alts);
+		if (r < 0) {
+			printk(KERN_ERR "usblp: can't set desired altsetting %d on interface %d\n",
+				alts, usblp->ifnum);
+			return r;
+		}
 	}
 
 	usblp->bidir = (usblp->protocol[protocol].epread != NULL);